In recent years polymeric separation membranes have widely been used for the preparation of ultrapure water for semiconductors in the electronic industry, the filtration, purification or removal of microorganisms in the medical, pharmaceutical or food industries, or the filtration of industrial waste water, etc., and the current trend is one of continuing expansion in the range and volume of their applications and use. For example, a separation membrane whose ion fractions and organic substances are scarcely eluted and which is excellent in heat and chemical resistance is in demand for the preparation of ultrapure water for the production of semiconductors, while at thermal or nuclear power plants a separation membrane excellent in heat resistance is in strong demand for the stable, longtime filtration of condensed water having temperatures exceeding 100 degrees C. Thus, from the viewpoint of membrane performance the demand for increased heat and chemical resistance is growing.
On the other hand, the materials which have widely been used heretofore for manufacturing separation membranes for ultrafiltration or microfiltration include cellulose derivatives such as cellulose acetate, etc., polyacrylonitrile resins, polyamide resins, polymethyl methacrylate resins, polysulfone resins, polyvinylidene fluoride resins, polyolefin resins, polycarbonate resins and the like. However, because of highly advanced requirements for separation membranes in recent years as mentioned above, these materials have been unsatisfactory with regard to their elution characteristics and the heat resistance and chemical resistance of the membranes made therefrom.
Attention is being paid to PEEK because of its excellent heat resistance, chemical resistance and low elution characteristics, so that this resin has been tried as a material for filtration membranes. For example, JP-A-61-115954 (equivalent to European patent 182506) describes a separation membrane of a sulfonated polyaryl ether ketone and a process for preparing the same. It is however known that such a membrane swells in water (Macromolecules, 86p, 18, 1985) and therefore degrades remarkably in mechanical strength and separation performance, for example, in water of 60 degrees C or higher.
Further, non-sulfonated PEEK membranes have been proposed. For example, JP-A-2-136229 (equivalent to U.S. Pat. No. 4,992,485) proposes a process for preparing a PEEK filtration membrane by using the membrane casting solution prepared by dissolving a specifically structured PEEK in a specific non-sulfonating acid solvent. However, the non-sulfonating acid solvents employed in this process are strong organic acids such as methane sulfonic acid, trichloromethane sulfuric acid, etc., hydrofluoric acid or mixtures of them and concentrated sulfuric acid. They are not only strong in toxicity and corrosiveness but also high in price and therefore are not suited for practical use from a commercial viewpoint. Further, these acids are disadvantageous for practical use because much work is required to make them harmless when they are recovered or disposed of as wastes.
Still further, JP-A-3-56129 (equivalent to European patent 382356) and JP-A-5-192550 (equivalent to EP-A-499381) describe methods of preparing asymmetric polyetherketone resin membranes, but these methods use the aforementioned specific strong acids such as methanesulfonic acid, hydrofluoric acid, etc. as solvents for PEEK.
The acid that may be easily used industrially to dissolve PEEK is concentrated sulfuric acid, which however has not been used in general as a solvent for PEEK because it sulfonates the phenylene groups between the ether groups of PEEK.